This invention relates to operation of a halogen lamp in a dental curing process and, more particularly, to the obtaining of more power in a usefull spectral region without diminishing useful lifetime of the lamp.
A halogen lamp has a tungsten filament which is heated by electric current during operation of the lamp. The lamp""s lifetime is based on the integrity of the filament. Once the filament burns out, the lamp no longer operates. While the lamp is operating, heat produced by electric current in the filament induces evaporation of the filament. Thus, the filament is continuously evaporating during operation of the lamp and, when the diameter of the filament reaches the critical point, the lamp burns out. A higher current flow in the filament results in a more rapid evaporation. Thus, during the start-up interval of the lamp, before the filament has attained its operating temperature, a much larger current flows with a resultant higher rate of evaporation.
In order to preserve the lifetime of a lamp having a tungsten filament during operation wherein the lamp may be turned on and off frequently, it is known to use a current limiter such as an NTC (negative temperature coefficient) thermistor to suppress the initial inrush current while providing negligible electrical resistance once the filament has heated to operating temperature. Unfortunately, such a current limiter develops significant heat after suppressing the initial inrush current, and requires a cool-town (recovery) time after power is removed from the lamp and prior to reactivation of the lamp. Due to the temperature dependent operating characteristic of the thermistor, the thermistor must be allowed to cool down in order to restore its resistance to the appropriate value for reactivation of the lamp. The cool-town time is approximately one minute. This presents an inconvenience in the situation wherein it is desired to turn the lamp off momentarily before reactivating the lamp. Also, the current limiter suffers from the disadvantage of dissipating some of the electric power which would otherwise be employed usefully in operation of the lamp.
In the use of a halogen lamp for curing dental material, it is advantageous to operate the lamp in a fashion which accelerates the rate of the curing. Thereby, the curing can be accomplished advantageously in a lesser amount of time. One way to accomplish this is to raise the operating voltage of the lamp, resulting in increased power dissipation in the lamp with increased light output. However, this introduces the disadvantage of reduced lifetime of the lamp. Another factor to consider in accelerating the curing process is the spectral distribution of light produced by the lamp. The curing process is accomplished best with a light wavelength of approximately 450 nm (nanometers). However, more than 95 percent of halogen lamp light, in the case of a typical halogen lamp heated by a tungsten filament, has a wavelength over 600 nm. Therefore, normal operation of a halogen lamp suffers from a lack of optimization of the spectral distribution of the light for use in the curing of dental material.
In view of the aforementioned disadvantages and problems, it is an object of the present invention to operate a halogen lamp in a manner which accomplishes a more rapid curing of dental material while substantially retaining the expected lifetime of the lamp. In accordance with a feature of the practice of the invention, it is recognized that the spectrum of light radiated by the lamp is dependent on the electrical voltage employed for exciting the lamp. Thus, an increase of the operating voltage by 10 percent shifts the color temperature to give a 6 percent color temperature rise, in the case of a typical halogen lamp heated by a tungsten filament. The increase of the operating voltage by 10 percent in this lamp also results in an increase of radiated light energy by 34 percent. The increase in color temperature is manifested by a shifting of the maximum spectral radiation output from longer wavelength toward shorter wavelength. In the case of the foregoing example, the spectrum shifting results in an additional 5 percent light output. While the increase in applied voltage provides the foregoing advantageous features in the operation of the lamp, it is noted that the increased voltage is accompanied by a disadvantage in that the foregoing 10 percent increase in the operating voltage results in a shortening of the lamp""s life by 40 percent.
In accordance with the methodology and the apparatus of the invention, enhanced usage of the halogen lamp for the curing of dental material is obtained by increasing the voltage applied to the lamp to shift the spectrum and obtain better utilization of the frequency spectrum of the output light, and to increase intensity of the output light, while regulating the applied voltage for a gradual increase of the voltage at the time of initial turn on of the lamp so as to avoid the initial surge current with its associated rapid evaporation of the filament. The design lifetime of the lamp is retained because the loss in use of the lamp associated with the increased voltage is balanced by the improved utilization of the spectrum, and the avoidance of the initial surge current so as to maintain an upper bound on the magnitude of the current.